1. Field of the Invention
This invention is related to the field of producing carbon nanotubes, and more particularly, but not by way of limitation, to methods of producing single-walled carbon nanotubes.
2. Brief Description of the Prior Art
Carbon nanotubes (also referred to as carbon fibrils) are seamless tubes of graphite sheets with full fullerene caps which were first discovered as multi-layer concentric tubes or multi-walled carbon nanotubes and subsequently as single-walled carbon nanotubes in the presence of transition metal catalysts. Carbon nanotubes have shown promising applications including nanoscale electronic devices, high strength materials, electron field emission, tips for scanning probe microscopy, and gas storage.
Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes for use in these applications because they have fewer defects and are therefore stronger and more conductive than multi-walled carbon nanotubes of similar diameter. Defects are less likely to occur in single-walled carbon nanotubes than in multi-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects.
However, the availability of these new single-walled carbon nanotubes in quantities necessary for practical technology is still problematic. Large scale processes for the production of high quality single-walled carbon nanotubes are still needed.
Presently, there are three main approaches for synthesis of carbon nanotubes. These include the laser ablation of carbon (Thess, A. et al., Science 273, 483 (1996)), the electric arc discharge of graphite rod (Journet, C. et al., Nature 388, 756 (1997)), and the chemical vapor deposition of hydrocarbons (Ivanov, V. et al., Chem. Phys. Lett 223, 329 (1994); Li A. et al., Science 274, 1701 (1996)). The production of multi-walled carbon nanotubes by catalytic hydrocarbon cracking is now on a commercial scale (U.S. Pat. No. 5,578,543) while the production of single-walled carbon nanotubes is still in a gram scale by laser (Rinzler, A. G. et al., Appl. Phys. A. 67, 29 (1998)) and arc (Haffner, J. H. et al., Chem. Phys. Lett. 296, 195 (1998)) techniques.
Unlike the laser and arc techniques, carbon vapor deposition over transition metal catalysts tends to create multi-walled carbon nanotubes as a main product instead of single-walled carbon nanotubes. However, there has been some success in producing single-walled carbon nanotubes from the catalytic hydrocarbon cracking process. Dai et al. (Dai, H. et al., Chem. Phys. Lett 260, 471 (1996)) demonstrate web-like single-walled carbon nanotubes resulting from disproportionation of carbon monoxide (CO) with a molybdenum (Mo) catalyst supported on alumina heated to 1200.degree. C. From the reported electron microscope images, the Mo metal obviously attaches to nanotubes at their tips. The reported diameter of single-walled carbon nanotubes generally varies from 1 nm to 5 nm and seems to be controlled by the Mo particle size. Catalysts containing iron, cobalt or nickel have been used at temperatures between 850.degree. C. to 1200.degree. C. to form multi-walled carbon nanotubes (U.S. Pat. No. 4,663,230). Recently, rope-like bundles of single-walled carbon nanotubes were generated from the thermal cracking of benzene with iron catalyst and sulfur additive at temperatures between 1100-1200.degree. C. (Cheng, H. M. et al., Appl. Phys. Lett. 72, 3282 (1998); Cheng, H. M. et al., Chem. Phys. Lett. 289, 602 (1998)). The synthesized single-walled carbon nanotubes are roughly aligned in bundles and woven together similarly to those obtained from laser vaporization or electric arc method. The use of laser targets comprising one or more Group VI or Group VIII transition metals to form single-walled carbon nanotubes has been proposed (WO98/39250) . The use of metal catalysts comprising iron and at least one element chosen from Group V (V, Nb and Ta), VI (Cr, Mo and W), VII (Mn, Tc and Re) or the lanthanides has also been proposed (U.S. Pat. No. 5,707,916). However, methods using these catalysts have not been shown to produce quantities of nanotubes having a high ratio of single-walled carbon nanotubes to multi-walled carbon nanotubes.
In addition, the separation steps which precede or follow the reaction step represent the largest portion of the capital and operating costs required for production of the carbon nanotubes. Therefore, the purification of single-walled carbon nanotubes from multi-walled carbon nanotubes and contaminants (i.e., amorphous and graphitic carbon) may be substantially more time consuming and expensive than the actual production of the carbon nanotubes.
Further, one of the greatest limitations in the current technology is the inability to obtain a simple and direct quantification of the different forms of carbon obtained in a particular synthesis. Currently, transmission electron microscopy (TEM) is the characterization technique most widely employed to determine the fraction of single-walled carbon nanotubes present in a particular sample. However, transmission electron microscopy can only provide a qualitative description of the type of carbon species produced. It is hard to determine how representative of the overall production a given transmission electron microscopic image can be. Obtaining semi-quantitative determinations of the distribution of different carbon species in a sample with any statistical significance is time consuming, and the method employing transmission electron microscopy could not be applied as a routine quality control to large-scale operations.
Therefore, new and improved methods of producing nanotubes which enable synthesis of commercial quantities of substantially pure single-walled carbon nanotubes and at lower temperatures than previously reported, as well as methods to directly quantify the different forms of carbon obtained in a particular synthesis, are being sought. It is to such methods of producing nanotubes and quantifying synthesis products that the present invention is directed.